Soil texture and salinity effects on calibration of TDR300 dielectric moisture sensor

Abstract

Newly developed sensors have simplified real-time determination of soil water content (θm). Although the TDR300 is one of the most recent dielectric sensors, little is known with regard to the accuracy and dependency of its measurements of soil type and other environmental factors. In this study, the performance of TDR300 was investigated using liquids of known dielectric properties and a set of porous media with textures ranging from sandy to clayey. The experiments were conducted in the laboratory by mixing different amounts of water with each soil to obtain a sufficient range of soil water contents. For sand, the calculated permittivity values (εr) correlated adequately with Topp’s equation derived for time domain reflectometry. However, for the remaining inorganic porous media, εr values were overestimated compared with those resulting from Topp’s equation, especially for water contents exceeding 0.2 cm3/cm3. The results suggested that the relationship between θm and √εr was strongly linear (0.953< r2 <0.998). The most accurate results were provided by soil-specific calibration equations, which were obtained by the multi-point calibration equation. However, two-point calibration equations determined water content in all tested soils reasonably well, except for clay soil. A linear regression equation was developed that correlated the slope of the relationship θm–√εr with bulk soil electrical conductivity (EC). The regression slope was influenced more by soil EC than by soil texture. Also, TDR300 response was investigated in bi-layered systems (liquid–air and saturated porous media–air). In a bi-layered sensing volume characterised by strongly contrasting dielectric values, the appropriate bulk permittivity values for water and loam soil were determined by arithmetic rather than refractive index averaging, while for butanol and sand these values remained somewhere between the two averaging schemes, indicating that the upward infiltration calibration technique is inappropriate for the TDR300 sensor. Soil solute EC, as determined by measurements conducted in liquids and sand, significantly affected permittivity values at much lower levels than the limit of EC <2 dS/m, as suggested by the manufacturer. However, the relationship θm–√εr remained linear up to EC 2 dS/m, which corresponded to a bulk soil EC value of 0.6 dS/m. By contrast, for EC values >2 dS/m, the relationship θm–√εr was not linear, and, thus the TDR300 device calibration became increasingly difficult. Therefore, rather than operating as a time domain device, TDR300 operates as a water content reflectometer type device.